A quantum cascade laser (QCL) is a multilayer semiconductor laser, based only on one type of carrier (usually electrons). A schematic diagram of a QCL is shown in FIG. 1. It consists of multiple layers of AlxIn1-xAs/InyGa1-yAs having different compositions x and y, typically grown using molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD) techniques. Electric current in these devices is injected along the x-axis, perpendicular to the grown layers. An insulator confines the current under the contact stripe, preventing it from spreading in the y-direction. When carriers reach the gain section, they emit photons through intersubband radiative transitions (see below). Waveguide and cladding layers confine emitted light around the gain region and direct it along the z-axis. A laser of this type is described in U.S. Pat. No. 5,457,709.
The gain section of a QCL usually consists of 20 to 60 identical gain stages. A gain stage consists of approximately 20 very thin InxGa1-xAs and AlyIn1-yAs layers (1-5 nm) with alternating bandgap material compositions (quantum wells and barriers). A schematic of the conduction band diagram of one gain stage under applied electric field is shown in FIG. 2. In an ideal case each carrier emits one photon in each gain stage.
As claimed in U.S. Pat. No. 5,457,709, the layers within the stage are usually divided into two regions: the active region and the energy relaxation region (injector). The active region is designed for light emission through carrier radiative intersubband transitions (transition from level 3 to level 2 in FIG. 2), while the energy relaxation region (injector) is used for energy relaxation of carriers before injection into the next stage.
Carrier population inversion between the upper and lower laser levels (levels 3 and 2 in FIG. 2), required for lasing, can be achieved when the upper laser level lifetime, τ3, is longer than the lower laser level lifetime, τ2. As claimed in U.S. Pat. No. 5,457,709, this condition is met when the energy spacing between levels 2 and 1, E21, is designed to be substantially equal to the energy of the longitudinal optical (LO) phonon (˜35 meV in case of InP-based QCLs). In this case τ2 and lower laser level population are substantially reduced. This scheme is often called the single-phonon design.
QCL performance can be substantially improved employing a so-called two-phonon resonance design (see U.S. Pat. No. 6,751,244) instead of the single-phonon resonance design described above. A schematic conduction band diagram for this design is shown in FIG. 3. The active region in this case is composed of at least four quantum well/barrier pairs instead of at least three for the single-phonon design. The lasing transition occurs between energy levels 4 and 3. Significantly, energy spacings E32 and E21 are both substantially equal to LO phonon energy, leading to short τ3 and τ2. Since the energy spacing between the lower laser level and the lowest active region level E31 (˜70 meV) is increased by a factor of two compared to E21 in case of the single-phonon resonance design (˜35 meV), the two-phonon resonance design has an advantage of reduced carrier population on the lower laser level 3 due to reduced carrier thermal backfilling for this state from the lowest active region state 1.
The last active region barrier (the so-called extraction barrier), in FIG. 3 that illustrates the two-phonon design, is usually thicker than the other active region barriers (except the first active region barrier, called the injection barrier). This helps to confine the lower laser level within the active region, increasing its overlap with the upper laser level and, as a consequence, increasing the laser transition matrix element. However, a thicker extraction barrier leads to longer electron extraction time from the active region to the injector, as discussed in Reference 1 (citation provided below). Longer extraction time increases global transit time of an electron across an active region stage, leading, in turn, to lower maximum current density for the same doping level. See Reference 2 (citation provided below) and references therein for details.
In the Bound-to-Continuum design, described in U.S. Pat. No. 6,922,427, the laser transition occurs between the upper laser level and delocalized lower laser levels, as shown in FIG. 4. Since the lower laser levels are delocalized, there is no electron extraction bottleneck that slows electron transport in the two-phonon case. However, since the laser transition is diagonal, the corresponding matrix element is lower and linewidth is larger. Both lower laser transition matrix element and larger linewidth reduce the active region differential gain, lowering laser performance.
The Single Phonon Resonance-Continuum Depopulation design, presented in Reference 3 (citation provided below), combines vertical laser transition, characteristic to the two-phonon design, and delocalized carrier extraction, characteristic to the bound-to-continuum design. However, this combination is realized only at bias close to roll-over, when the lowest injector state in the previous stage and the upper laser level are close to resonance. Indeed, according to the band diagram presented in Reference 3, calculated approximately at roll-over bias, there is an injector level located just below the lower laser level. Therefore, at slightly lower bias, these levels align with each other due to the Stark effect. Since there is a strong active region/injector coupling, these levels become delocalized at lower bias. As follows from typical QCL voltage vs. current (IV) characteristics, current starts flowing through a superlattice at bias significantly below its roll-over value. Therefore, delocalization of the lower laser level leads to an increase in threshold current density, reducing laser dynamic range, maximum optical power and wall plug efficiency.
The goal of the present invention is to achieve vertical transition and fast carrier extraction in a broad bias range, i.e. to introduce a bias independent design method. Stability of QCL parameters at lower bias has been consistently overlooked in QCL design so far.